JP-2012-005252A (US 2011/0309707 A1) discloses a consequent-pole type motor in which half of magnetic poles are soft magnetic poles such that an amount of rare earth element is reduced. In this case, the rare earth element corresponds to a material of a permanent magnet included by a rotor used as a permanent magnet field. A rotor core and a stator core form a main magnetic circuit for generating a torque. It is well-known that a magnetic flux generated by the permanent magnet of the rotor is readily leaked from the main magnetic circuit. According to JP-2012-005252A, a distance of a gap disposed in an axial direction between a center portion of a bottom portion of a bottomed cylindrical case and the rotor core, is at least equal to and is at most ten times as a distance of a gap disposed in a radial direction between the stator core and the soft magnetic pole of the rotor.
A rotary position detecting portion, which uses a semiconductor magnetic sensor and a detecting magnet instead of using a resolver corresponding to a rotary position detector of a magnetic modulation type, is used in a control of a brushless motor. In this case, the semiconductor magnetic sensor may be a MR sensor including a magnetoresistive element, and the detecting magnet is used as a source for generating a detected signal. When the semiconductor magnetic sensor is disposed at a position along a rotary shaft of the brushless motor, a magnetic flux leaked from the motor disturbs the semiconductor magnetic sensor, and an accuracy of detecting a rotary position of the semiconductor magnetic sensor may deteriorate. According to JP-2012-005252A, a gap between the bottom portion of the bottomed cylindrical case and the rotor core is established to reduce the magnetic flux leaked from the bottomed cylindrical case toward the semiconductor magnetic sensor.
However, when the gap between the bottom portion of the bottomed cylindrical case and the rotor core is excessively small, a magnetic resistance of the gap decreases so that the magnetic flux passing through the bottomed cylindrical case increases. Therefore, the center portion becomes magnetically saturated, and the magnetic flux leaked from the bottomed cylindrical case toward the semiconductor magnetic sensor increases. The magnetic flux generated by the permanent magnet and flowing in an axial direction of a cylindrical portion of the bottomed cylindrical case flows radially in the bottom portion of the bottomed cylindrical case, and joins together at the center portion. Since the center portion has a magnetism-passage sectional area less than that of the cylindrical portion, the center portion is readily magnetically saturated.
When the bottom portion is formed by stamping to have a thickness increasing in accordance with a decrease in distance to the center portion, the center portion can be prevented from becoming magnetically saturated. However, it is difficult to use stamping to form the bottom portion. Therefore, a cost may increase.